Secondary electron conduction storage system



April 29, 1969 G. w. GOETZE ETAL 3,441,787

SECONDARY ELECTRON CONDUCTION STORAGE SYSTEM Filed April 27. 1967 F|G.l.

WITNESSES INVENTORS WW Gerhard W. Goetze 0nd Alvin H. Boerio U.S. Cl.31512 4 Claims ABSTRACT 9F THE DISCLOSURE A secondary electronconduction target storage tube in which the secondary electronconduction material is disposed as a porous material upon a mesh supportmember and which fills the interstices of the mesh and covers both sidesof the mesh.

Background of the' invention This invention relates to secondaryelectron conduction storage systems and more particularly to those whichincorporate a secondary electron conduction target.

The particular application of this invention is to that type of devicedescribed in U.S. Patent 3,213,316 and which is assigned to the sameinventors and assignee as this invention. The above-mentioned patentdescribes a secondary electron conduction tube including a targetstructure which consists of a thin film of aluminum oxide as a supportlayer stretched across a metal ring. A thin evaporated layer of aluminumwhich serves as a signal electrode is disposed upon this support layerand a layer of a suitable porous secondary electron conduction materialis disposed on the signal electrode. A writing electron beam is directedinto the secondary electron conduction layer through the aluminum oxideand aluminum layers and a reading beam is directed onto the exposedsurface of the secondary electron conduction layer. It is also necessaryin most devices to provide a conductive mesh in front of the exposedsurface of the secondary electron conduction layer which serves as asuppressor grid and prevents destruction of the target under certainoperation conditions.

It has been found that considerable skill and time are required infabricating this multi-layer thin film structure. There are severalproblems inherent in manufacturing and operating the thin film structureas described in the above-mentioned patent. The diameter of the targetsare limited to less than two inches. It is difficult to recognizedefects found in the base supporting material of aluminum oxide whichresults in target blemishes. The target is not a rugged structureparticularly in the larger type tubes. The porous secondary electronconduction layer must be evaporated in an inert gas atmosphere and timevarying convection currents induced by the evaporation make it extremelydifiicult to reproduce desired target structures. The non-uniformity ofdeposit particularly affects the target capacity and the lagcharacteristics of the structure. The storage capacity of the targetdepends on the target voltage and those cases where automatic targetgain control is used, this dependence is highly undesirable.

It is accordingly an object of this invention to provide an improvedsecondary electron conduction target.

It is another object to provide an improved secondary electronconduction tube.

3,441,787 Patented Apr. 29, 1969 Summary of the invention Briefly, thepresent invention accomplishes the abovecited objects by providing asecondary electron conduction target utilizing a conductive mesh as thesupport for the second electron conduction material and in which theconduction material is deposited within the interstices of the mesh andon both sides thereof.

These and other objects and advantages of the present invention willbecome more apparent when considered in view of the following detaileddescription and drawings, in which:

Brief description of the drawing FIGURE 1 is an elevational view insection, schematically representing the pickup tube and associatedsystem in accordance with the teachings of this invention; and

FIG. 2 is an enlarged elevational view in section illustrating theelectrode target assembly in FIG. 1.

Descriptions of the preferred embodiment Referring in detail to FIGS. 1and 2, there is illustrated a pickup tube in FIG. 1 incorporating theteachings of our invention. The tube comprises an envelope 10. A faceplate 12 is provided in the envelope 10 and is transmissive to the inputscene radiations. The face plate 12 is of a suitable material such asglass in the case of visible light input. A coating 14 of a suitablephotoemissive material which is sensitive to the input radiation isprovided on the inner surface of the face plate 12. The coating 14 emitsphotoelectrons in response to the input radiation. A suitable materialfor the coating 14 in the case of visible light input would be cesiumantimony.

An electron gun 20 is provided at the opposite end of the envelope 10for generating and forming a pencil-like electron beam which is directedonto a target member 30. The target member 30 is positioned between theelectron gun 20 and the photocathode 14. Between the target member 30and the photocathode 14, there are provided a plurality of electrodesillustrated as 16 and 18 with suitable potentials provided thereon foraccelerating and focusing of the photoelectrons emitted from thephotocathode 14 onto the target member 30. The electron gun 20 provideswhat is known in the art as a reading electron beam and the photocathode14- provides what is referred to in the art as the writing electronbeam. In the specific examples shown herein a large area image inputphotocathode is utilized. It is obvious that modifications could be madeherein so as to make the photosurface sensitive to various inputradiations. This could be accomplished by a radiation converter in whichthe input radiation is directed onto a phosphor and the light output ofthe phosphor is directed onto the photocathode 14. In addition, aconventional scanning electron gun could be utilized for directing anelectron beam over the target surface in a similar manner as the readingelectron gun 20. The video information in this case would be applied tothe writing electron gun in conventional manner.

The electron gun 20 is of any suitable type for producing a low velocitypencillike electron beam to be scanned over the surface of the targetelectrode 31). The electron gun may consist of a cathode 22, a controlgrid 24 and accelerating grid 26. The gun electrodes 22, 24 and 26 alongwith a coating 44 provided on the inner wall of the envelope provide afocused electron beam which is directed onto the target member 30. Itmay be desirable in some applications to utilize a mesh screen 40 infront of the target in a well known manner for maintaining a uniformelectric field in front of the target 30. Deflection means illustratedas a coil 50 is provided around the envelope 10 for deflection of theelectron beam generated by the electron gun 2t) and by application ofsuitable potentials scans the electron beam over the surface of thetarget 30 in a suitable manner. A magnetic coil 52 is also providedaround the envelope 10 to provide additional focusing of the electronbeam from the read gun 20 onto the target 30 as well as for focusingelectrons from the photocathode 14 onto the target 30.

The target member 30 is supported upon a ring member 32 of a suitablematerial such as a Kovar alloy, a Westinghouse Electric Corporationtrademark for an alloy of nickel, iron and cobalt. The ring 32 issupported within the envelope by any suitable means such as pinsprojecting through the glass walls. Further, the storage target 30 iscomprised of a very fine mesh 34 which is made of a suitableelectrically conductive material such as copper or nickel and which issecured to the ring 32 by an annular support member 36 which may be spotwelded to the ring 32. The mesh 34 may be of the woven type or may bemade from a solid sheet which has been etched to provide a perforatedstructure. In the specific embodiment illustrated, the mesh 34 has about1000 openings per inch and an open area of 50 percent or more. Theopenings in the mesh are about 12 micrometers by 12 micrometers. Theindividual mesh element width is about 6 to 12 micrometers. Thethickness of the mesh 34 may be about 6 to 20 micrometers. A layer 38 ofa material exhibiting secondary electron conduction is depositeddirectly onto the mesh 34. Suitable examples of materials which exhibitthis secondary electron conduction property includes potassium chloride,barium fluoride, sodium bromide and magnesium oxide. The layer 38 is aspongy or porous deposit having a density of less than 10 percent of thedensity of the material in its normal state. The porous layer 38 isformed by the evaporation of the secondary electron conduction materialonto the mesh 34. The material to be deposited is heated to itsevaporation temperature in the presence of an inert atmosphere, forexample, helium or argon. The evaporation takes place at a distance inthe order of a few inches in an atmospheric pressure of about 1 to torr.It is an important aspect of this invention that portions of theelements of the porous material are disposed within and over thesurfaces of the mesh 34 to provide the layer 38. The material may beevaporated from both sides of the mesh simultaneously or may beevaporated sequentially in any suitable manner. The deposit Will growsideways from the interstices of the mesh 34 to thereby fill in theinterstices and then cover the entire area of the mesh and provide acontinuous layer 30 of the porous material on both surfaces of the mesh34. If the thickness of the mesh 34 is about 12 micrometers then thethickness of the target after the coating 38 is deposited is about 20micrometers. This provides a porous coating of about 4 micrometers oneach side of the mesh. The porous layer on each side of mesh may be of 4to 20 micrometers. The active portion of the mesh 34 is completelyembedded within the layer 38.

The values of representative potentials applied to the electrodes areillustrated in FIG. 1. The photocathode 14 is operated at a potential ofabout 8,000 volts negative with respect to the conductive mesh 34 toprovide acceleration of the electrons from the photocathode 14 onto thetarget 30. The conductive mesh 34 may be operated at a potential ofabout 15 volts positive with respect to the electron gun cathode 22. Thecathode 22 operates at about ground potential. The surface of the porousstorage coat ing 38 is stabilized on the read side to an equilibriumpotential which may be substantially ground potential by means of thescanning electron beam from the gun 20. If the grid 40 is utilized infront of the target 30- and between the electron gun 20 and the target30, a retarding field will exist between the target 30 and the grid 40.Such a grid 40 would be operated at a potential of about 450 voltspositive with respect to ground. In addition a suppressor grid 41 may beprovided between the grid 48 and the target 30. The mesh 41 may operateat a potential of positive 50 volts.

The radiations from a scene are focused onto the pho tocathode 14 andphotoelectrons are emitted from each portion of the photocathode 14corresponding to the amount of light directed thereon. Thephotoelectrons are focused upon the target member 30. The photoelectronsare accelerated to sufficiently high energy of about 5000 electron voltsso that they penetrate into the coating 38. The accelerating voltageshould be adjusted such that substantially all of the primary electronsfrom the photocathode 14 almost completely penetrate the layer 38 but donot substantially pass on through the structure. The primary electronsfrom the photocathode 14 create a certain number of low energy or freeelectrons within the layer 38. The number of low energy electronsgenerated are orders of magnitude higher than the number of primaryelectrons. For example, the number of free electrons generated may beabout 200 for each incident primary. The target 30 is polarized prior tothe impact of the signal or writing electrons from photocathode 14. Thisis done by applying a positive potential of about 15 volts to theconductive mesh 34 and stabilizing the exposed surface or read side ofthe target at ground potential and the free electrons generated in thelayer 38 flow within the voids of the layer 38. This causes the readsurface to change its potential from ground due to flow of the freeelectrons in the layer 38 through the vacuum space or voids between theparticles of the very porous layer 38 to the positive backplate or mesh34. This local change of the exit surface potential that is on the readside can be employed to generate a video signal using any of the severalwell known readout techniques. In FIG. 1 there is illustrated a typicalvidicon type readout assembly.

The storage capacity of this novel target 30 is determined by thegeometry of the mesh 34 chosen and does not therefore depend as stronglyon evaporation parameters and the time variance convection currents.This feat-ure makes it possible to predetermine and accurately controlthe target storage capacity required by the target format. Since no welldefined plane serves as the signal plate, the undesired dependence ofstorage capacity on target voltage is in the first order eliminated.This will make it possible to use the target voltage as a parameter inachieving gain control.

Various modifications may be made within the spirit of the invention.

We claim as our invention:

1. An electron discharge device comprising a storage electrode includingan electrically conductive mesh support member, a porous film deposit ofless than 10 percent of its normal bulk density of high resistivematerial deposited in the interstices of said mesh and upon the surfacesof both sides of said mesh and having the property of generating freeelectrons in response to electron bombardment, means for directing awriting electron beam at one side of said target at a predeterminedenergy to penetrate said porous film to generate secondary elec tronswithin said porous film, means for establishing a field across saidporous film to collect said secondary electrons emitted into the vacuumspaces within the particles of said porous layer but inadequate tocollect charge carriers through said solid material and means fordirecting a reading electron beam below said predetermined energy at theother surface of said storage target to restore said bombarded surfaceto an equilibrium potential while deriving a signal corresponding to thesignal written on said storage target by said writing beam.

2. The electron discharge device described in claim 1 in which theinterstices and said storage mesh are of similar dimensions as thethickness of said porous coating on one surface of said conductive mesh.

3. The electron discharge device described in claim 1 in which saidconductive mesh has openings of 500 to 1000 per inch constituting anopen area of 50% or more and the thickness of said porous coatings oneach surface of said mesh is about 10 micrometers.

4. The electron discharge device described in claim 1 in which saidconductive mesh has a thickness of about 20 micrometers with about 1000openings per inch, each opening of a dimension of about 12 micrometersby 12 micrometers with the porous coating filling the mesh openings andextending beyond the mesh on the reading side of the target.

6 References Cited UNITED STATES PATENTS RODNEY D. BENNETT, JR., PrimaryExaminer.

CHARLES L. WHITHAM, Assistant Examiner.

US. Cl. X.R.

